This invention relates to an exposure amount control method, a device manufacturing method, and/or an exposure apparatus, suitably applicable to a projection exposure apparatus, for example, to be used in a lithographic process for the manufacture of semiconductor devices, liquid display devices, image pickup devices (e.g., CCDs), or thin film magnetic heads, for example. The present invention is particularly suitably applicable to a scanning exposure type projection exposure apparatus of a step-and-scan method wherein a mask and a substrate are scanningly moved in synchronism with each other and relative to a projection optical system while a portion of the mask pattern is being projected onto the substrate, by which the mask pattern is sequentially transferred to shot regions on the substrate.
FIG. 1 shows a conventional scan type projection exposure apparatus having an illumination unit for uniformly illuminating a pattern by use of light from an exposure light source, and a projection optical system for projecting the thus illuminated pattern onto a substrate. While the following description will be made on an example wherein a divergent light source such as a Hg lamp, for example, is used, the same applies to an example wherein a light source has a directionality such as a laser, for example. Denoted in FIG. 1 at 1 is a light source, and denoted at 2 is a condensing mirror for collecting light emitted from the light source 1. The light divergently emitted from the light source 1 is collected by the mirror 2, and the light then passes through an optical system 3. Then, it goes through a light attenuating unit 18 and enters an internal reflection member 4.
The internal reflection member 4 has a structure that the light incident thereon is reflected plural times at the side face thereof, whereby the light intensity distribution being non-uniform at the light entrance surface is made uniform at the light exit surface thereof. The internal reflection member 4 may be mirrors disposed opposed to each other, or it may be simply a rod-like glass material. In the latter case, it should be designed so that, when the light impinges on the side face of the rod-like glass material, the light is totally reflected due to the difference in refractive index between the glass material and the air, and that the side face should be polished.
Denoted at 5 is an optical system for projecting a uniform intensity distribution, produced at the light exit surface of the internal reflection member 4, onto a light entrance surface of a fly""s eye lens 6. The fly""s eye lens 6 comprises a bundle of rod lenses each having a light entrance surface and a light exit surface being mutually placed at their focal points. Light rays incident on the rod lenses at the same angle are collected at the light exit surfaces of the rod lenses, such that, in total, a number of light convergence points are defined at the light exit surface of the fly""s eye lens 6.
Denoted at 7 is an optical system for uniformly illuminating an illumination region by using, as secondary light sources, the light convergence points produced at the light exit surface of the fly""s eye lens 6. In projection exposure apparatuses, it is necessary to control the illumination region and, for this reason, the illumination region is not directly illuminated but a uniform light intensity distribution is provided once at a position optically conjugate with the mask surface. Denoted at 8 is a stop for controlling the illumination region, and the position thereof is placed optically conjugate with the mask surface. A uniform light intensity distribution produced there is projected by a relay optical system 9 to illuminate a mask 10 (illumination region).
The stop 8 for controlling the illumination region is made movable, and it can be moved in accordance with the illumination region on the mask 10. Only a set of stop components is illustrated in the drawing, but actually it comprises at least two sets of stop components. One set of them is used to restrict the exposure region on a substrate, and it moves in synchronism with the substrate at the start and the end of the exposure region. Another set is used to control the illumination region of the illumination light, and it is held fixed during the exposure. This stop is driven so as to change the width of the illumination region in the scan direction to control the exposure amount, or to change the width at each position so as to remove exposure non-uniformness in the direction perpendicular to the scan direction.
Denoted at 11 is a projection optical system for imaging a pattern, formed on the mask 10, onto a substrate 12. It serves to print the pattern of the mask 10, as illuminated with uniform illumination light provided by the above-described illumination system, on a photosensitive material applied to the substrate 12 thereby to transfer the mask pattern to the substrate. The projection optical system 11 is a telecentric system such that, even with a shift of the mask 10 position or substrate 12 position in the optical axis direction, the projection magnification is unchanged. The exposure apparatus is a scan type projection exposure apparatus and, thus, the mask 10 and the substrate 12 are scanningly moved in synchronism with each other. Also, because the mask 10 moves, the illumination region changes accordingly and, therefore, the stop 8 for controlling the illumination region is also made movable.
Denoted at 13 is a movable stage on which an illuminance sensor 14 is mounted. This stage is arranged to provide scan motion for scan exposure of the substrate 12, and stepwise motion for exposures of plural shots on the substrate. Further, it is operable to move the illuminance sensor 14 to the same position where the substrate 12 is present during the exposure process, to ensure that the sensor measures the illuminance at the same position as the substrate.
In projection exposure apparatuses for use in the manufacture of semiconductor devices, for satisfactory transfer of the pattern of the mask 10 onto the substrate 12, it is necessary to expose the substrate 12 with a proper exposure amount which is dependent upon the photosensitive material applied to the substrate 12 and the pattern of the mask 10. For example, when a positive pattern and a negative resist are used, an exposure with an amount less than a proper exposure amount causes insufficient printing, which leads to thinning of pattern lines or disconnection of the lines. On the other hand, an exposure amount with an amount larger than the proper exposure amount causes excessive printing, which leads to thickening of pattern lines or connection of adjacent lines. When a negative pattern and a positive resist are used, an exposure with an amount less than a proper exposure amount causes insufficient exposure, which leads to thickening of pattern lines or connection of adjacent lines. On the other hand, an exposure with an amount larger than the proper exposure amount causes excessive printing, which leads to thinning pattern lines or disconnection of lines. Anyway, an exposure with an improper exposure amount results in failure of formation of an adequate pattern on the substrate, which directly leads to a decrease of the yield.
The exposure amount in such a scan type projection exposure apparatus is xe2x80x9csxc3x97I/vxe2x80x9d where s is the length of the illumination region in the scan direction, I is the illuminance on the substrate 12, and v is the scan speed. Thus, for the control of the exposure amount in the scan type projection exposure apparatus, at least one of the length s of the illumination region in the scan direction, the illuminance I on the substrate, and the scan speed v should be controlled. The length s of the illumination region in the scan direction can be changed by moving the stop 8 for controlling the illumination region. The scan speed v can be changed by changing the scan speeds of the substrate 12, the mask 10 and the stop 8 for controlling the illumination region.
The light attenuating unit 18 is disposed on the light path, and the transmission factor thereof is made variable. It controls the illuminance I on the substrate 12 to provide a predetermined illuminance thereon. As for such light attenuating means having a variable transmission factor, an example may be one which includes plural optical members having different transmission factors which may be placed upon a turret and, by selecting these members, the transmission factor may be changed. Another example may be a unit in which a mirror whose reflection factor is variable in dependence upon the angle with the light and in which the angle of the mirror is changed to change the transmission factor. In practical projection exposure apparatuses, the output of the light source 1 involves flicker and, thus, without constant illuminance control based on illuminance measurement during the exposure operation, the exposure amount within the exposure region on the substrate becomes non-uniform and, therefore, non-uniform exposure results.
Constant illuminance control should be done to avoid such exposure nonuniformness. However, it is not attainable to perform the constant illuminance control by directly measuring the illuminance on the substrate 12 during the period for transferring the mask 10 pattern onto the substrate. Further, if the illuminance is measured at the light path of exposure light, a shadow of an illuminance sensor will be formed on the substrate 12. This adversely influences the transfer of the mask 10 pattern to the substrate 12. In consideration of it, generally, the illuminance is measured at a position which is conjugate with the substrate 12 and which is branched from the light path of the exposure light, and the exposure amount control is made on the basis of it.
Denoted at 15 is a half mirror having a very low reflection factor, which is inserted into the light path so as to define a position branched off the light path of the exposure light and being conjugate with the substrate 12. The half mirror 15 is disposed obliquely to the optical axis of the exposure light, and it functions to divide the exposure light and to define a position, where an illuminance sensor 16 for measuring the illuminance is present, which position is branched off the exposure light and is conjugate with the substrate 12. Measuring the illuminance at a position conjugate with the substrate 12 is to assure one-to-one correspondence with the illuminance on the substrate 12 regardless of variation with time of the light emission center of the light source 1, such as fluctuation, for example.
For exposure of the substrate 12 with a proper exposure amount, before the exposure of the substrate 12, the illuminance sensor 14 mounted on the stage 13 is moved to the illumination region for calibration thereof. The calibration is performed by executing a dummy exposure and then comparing outputs of the illuminance sensors 14 and 16. For exposure of the substrate 12, the control unit 17 operates on the basis of the output of the illuminance sensor 16 to perform the constant illuminance control.
Denoted at 19 is a shutter for opening and closing the light path of exposure light. In projection exposure apparatuses of simultaneous exposure type, the shutter is closed at the moment when the exposure amount on the substrate reaches the proper exposure amount, and the exposure amount is so controlled. In scan type projection exposure apparatuses, on the other hand, the exposure cannot be completed until the scan is completed. Thus, the exposure amount cannot be controlled on the basis of shutter opening and closure. In the scan type projection exposure apparatus, the shutter 19 is used to block the illumination light, during the stepwise motion between adjacent shots or a period in which no exposure process is performed, to thereby prevent degradation of the illumination system.
When a half mirror is used in the optical system, as the angle defined between the light and a normal to the half mirror increases, the difference in reflection factor depending on the state of polarization of the light becomes large. If, therefore, the angle between the optical axis and a normal to the half mirror 15 in FIG. 1 is large, the light intensity after the half mirror 15 is different in dependence upon the state of polarization. This results in non-uniform illuminance upon the illumination region. Further, if the state of polarization of the light source 1 changes with time, it causes variation in the ratio between the quantity of light impinging on the illuminance sensor and the quantity of light impinging on the substrate 12. As a result, the exposure amount control precision is degraded. Thus, in order that uniform illumination is performed in the illumination region and good exposure amount control is performed, the half mirror 15 should be disposed with an angle close to a right angle, as much as possible. Simultaneously, it should be assured that the light reflected by the half mirror 15 reaches the illuminance sensor 16 without being eclipsed. However, if the half mirror 15 is disposed on the light path while satisfying these two conditions, the spacing between the half mirror 15 and a lens just before the half mirror 15 must be kept large. This means that a large space should be prepared for the exposure amount control on the substrate 12.
On the other hand, for performance improvements as recently required for projection exposure apparatuses, an optical system of the projection exposure apparatuses is very complicated and the size thereof is increasing. In consideration of this, for reduction in size of the projection optical system, the space necessary for the exposure amount control should desirably be reduced. However, reducing this space will lead to non-uniform illuminance or degradation of the exposure amount control precision, as described hereinbefore.
It is accordingly an object of the present invention to provide an exposure amount control method, a device manufacturing method and/or an exposure apparatus, by which the space necessary for the exposure amount control can be reduced without non-uniform illuminance or degradation of the exposure amount control precision.